Polypropylene stretched films have been used in wide variety of fields, including food packaging, textile packaging etc., by virtue of their desirable transparency and mechanical properties.
Although polypropylene stretched films have such desirable properties, they are not entirely free from defects. The major problems are insufficient slip property and blocking resistance.
Various methods have already been proposed to improve the insufficient slip property and blocking resistance of polypropylene stretched films. Such known methods include a method for improving blocking resistance by the addition of zeolite, magnesium silicate, etc. (for example, JP-B-52-16134 and JP-B-48-14423), a method for improving transparency, slip property and blocking resistance by the addition of fine powdery silica (for example, JP-B-63-58170 and JP-A-4-288353), and a method for improving transparency, slip property and blocking resistance by the addition of a thermoplastic synthetic resin which is substantially incompatible with polypropylene (for example, JP-B-50-36262).
However, these methods of adding such inorganic fillers as zeolite, magnesium silicate, fine powdery silica, etc. are not always satisfactory. Thus, increasing the amount of such fillers incorporated into a polypropylene film in order to attain sufficient improvement of the slip property and blocking resistance of the film generally reduces the transparency of the film due to the development of voids caused by the action of the inorganic fillers as the nuclei. As a result, market value of the film is disadvantageously lowered. On the other hand, the addition of the thermoplastic synthetic resin accompanies a problem of unsatisfactory thermal stability of the resin in the course of melt-kneading.